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2 Operational Theory Model 3000PA
Teledyne Analytical Instruments
In the presence of oxygen the cell generates a current. A current to
voltage amplifier converts this current to a voltage, and then the voltage is
amplified in the second stage amplifier.
The second stage amplifier also supplies temperature compensation for
the oxygen sensor output. This amplifier circuit incorporates a thermistor,
which is physically located in the cell block. The thermistor is a temperature
dependent resistance that changes the gain of the amplifier in proportion to
the temperature changes in the block. This change is inversely proportional
to the change in the cell output due to the same temperature changes. The
result is a signal that is temperature independent. The output from the second
stage amplifier is sent to an 18 bit analog to digital converter controlled by
the microprocessor.
The digital concentration signal along with input from the control panel
is processed by the microprocessor, and appropriate control signals are
directed to the display, alarms and communications port. The same digital
information is also sent to a 12 bit digital to analog converter that produces
the 0-1 V dc analog percent-of-range signal output and the analog range ID
output. Models with the –MA option also have a 4-20 mA dc percent-of-
range signal output and analog range ID output.
Signals from the power supply are also monitored by the microproces-
sor, and the system failure alarm is activated if a malfunction is detected.